This invention relates to the preparation of diary carbonates by carbonylation. More particularly, it relates to the improvement of diary carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply xe2x80x9ccarbonylationxe2x80x9d for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
The production of carbonates may be improved by including a metal-based cocatalyst along with the heavy Group VIII metal catalyst. Although the identity of suitable metal-based cocatalysts will depend on specific reaction conditions including the identity of reactants and other members of the catalyst package, some general guidance can be found in U.S. Pat. Nos. 4,187,242 and 4,201,721.
A further development in the carbonylation reaction, including the use of specific lead compounds as cocatalysts, is disclosed in U.S. Pat. No. 5,498,789. Also required according to that patent is the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package. Compounds characterized as inert solvents, such as toluene, diethyl ether, diphenyl ether and acetonitrile, can also be present.
The commercial viability of the carbonylation reaction would be greatly increased if a less expensive compound could be substituted for the quaternary ammonium or phosphonium halide. Substitution of such compounds as sodium bromide, however, result in the isolation of the desired diaryl carbonate in low or insignificant yield.
It is of interest, therefore, to develop catalyst systems which include an inexpensive halide compound and which can efficiently produce diaryl carbonates. Some such systems are known. Reference is made, for example, to Japanese Kokai 10/316,627, which discloses the use of palladium and a lead or manganese compound in combination with a halide such as sodium bromide and with an amide or alkylurea. U.S. Pat. No. 5,726,340 and Japanese Kokai 9/278,716 disclose similar systems in which the lead is combined with another metal and in which inert solvents such as those mentioned hereinabove may be present. The development of other systems employing relatively inexpensive halides, however, remains desirable.
The present invention provides a method for preparing diaryl carbonates which includes a relatively inexpensive halide and a compound which maximizes the effectiveness of said halide. Also provided are catalyst compositions useful in such a method.
In one of its aspects, the invention provides a method for preparing a diaryl carbonate which comprises contacting at least one hydroxyaromatic compound with oxygen and carbon monoxide in the presence of an amount effective for carbonylation of at least one catalytic material comprising:
(A) a Group VIII metal having an atomic number of at least 44 or a compound thereof,
(B) at least one alkali metal halide or alkaline earth metal halide, and
(C) at least one polyether.
Another aspect of the invention is catalyst compositions comprising components A, B and C as described above, and any reaction products thereof.
Any hydroxyaromatic compound may be employed in the present invention. Monohydroxyaromatic compounds, such as phenol, the cresols, the xylenols and p-cumylphenol, are generally preferred with phenol being most preferred. The invention may, however, also be employed with dihydroxyaromatic compounds such as resorcinol, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane or xe2x80x9cbisphenol Axe2x80x9d, whereupon the products are polycarbonate oligomers.
Other reagents in the method of this invention include oxygen and carbon monoxide, which can react with the phenol to form the desired diaryl carbonate. They may be employed in high purity form or diluted with another gas such as nitrogen, argon, carbon dioxide or hydrogen which has no negative effect on the reaction.
For the sake of brevity, the constituents of the catalyst system are defined as xe2x80x9ccomponentsxe2x80x9d irrespective of whether a reaction between said constituents occurs before or during the carbonylation reaction. Thus, the catalyst system may include said components and any reaction products thereof.
Component A of the catalyst system is one of the heavy Group VIII metals, preferably palladium, or a compound thereof. Thus, useful palladium materials include elemental palladium-containing entities such as palladium black, palladium/carbon, palladium/alumina and palladium/silica; palladium compounds such as palladium chloride, palladium bromide, palladium iodide, palladium sulfate, palladium nitrate, palladium acetate and palladium 2,4-pentanedionate; and palladium-containing complexes involving such compounds as carbon monoxide, amines, nitrites, phosphines and olefins. Preferred in many instances are palladium(II) salts of organic acids, most often C2-6 aliphatic carboxylic acids, and palladium(II) salts of xcex2-diketones. Palladium(II) acetate and palladium(II) 2,4-pentanedionate are generally most preferred. Palladium(II) acetylacetonate is also a suitable palladium source. Mixtures of the aforementioned palladium materials are also contemplated.
Component B is at least one alkali metal or alkaline earth metal halide, preferably a bromide such as lithium bromide, sodium bromide, potassium bromide, calcium bromide or magnesium bromide. Alkali metal bromides are especially preferred, with sodium bromide often being most preferred by reason of its particular suitability and relatively low cost.
Component C is at least one polyether; i.e., at least one compound containing two or more Cxe2x80x94Oxe2x80x94C linkages. The polyether is preferably free from hydroxy groups to maximize its desired activity and avoid competition with the hydroxyaromatic compound in the carbonylation reaction.
The polyether preferably contains two or more (Oxe2x80x94Cxe2x80x94C) units. The polyether may be xe2x80x9caliphaticxe2x80x9d or mixed aliphatic-aromatic. As used in the identification of the polyether, the term xe2x80x9caliphaticxe2x80x9d refers to the structures of hydrocarbon groups within the molecule, not to the overall structure of the molecule. Thus, xe2x80x9caliphatic polyetherxe2x80x9d includes heterocyclic polyether molecules containing aliphatic groups within their molecular structure. Suitable aliphatic polyethers include diethylene glycol dimethyl ether (hereinafter xe2x80x9cdiglymexe2x80x9d), triethylene glycol dimethyl ether (hereinafter xe2x80x9ctriglymexe2x80x9d), tetraethylene glycol dimethyl ether (hereinafter xe2x80x9ctetraglymexe2x80x9d), polyethylene glycol dimethyl ether and crown ethers such as 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane) and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane). Illustrative mixed aliphatic-aromatic polyethers are diethylene glycol diphenyl ether and benzo-18-crown-6.
In a highly preferred embodiment of the invention, there is also present in the catalyst system (D) at least one cocatalyst which is a compound of a metal other than a heavy Group VIII metal. This metal is preferably one which is soluble in the liquid phase under the reaction conditions. Numerous other metal compounds are known in the art to be active as carbonylation cocatalysts, and any compound having such activity may be used according to the present invention provided an improvement in diphenyl carbonate production, usually yield, is achieved thereby.
Illustrative cocatalytic metals include cerium, titanium, cobalt, copper, zinc, manganese, iron and lead, which may be used singly or in combination. For the purposes of this invention the preferred cocatalysts are those containing metals other than Group VIII metals; that is other than iron, cobalt and nickel. More preferred are compounds of lead, particularly when used alone or in combination with titanium and/or cerium. It should be noted, however, that component C is not effective to optimize diaryl carbonate formation for all possible permutations of component D; the combined effectiveness of the two for this purpose may be determined by simple experimentation.
Examples of lead compounds which may be employed are lead oxides such as PbO and Pb3O4; inorganic lead salts such as lead(II) nitrate; lead carboxylates such as lead(II) acetate and lead(II) propionate; lead alkoxides and aryloxides such as lead(II) methoxide and lead(II) phenoxide; and lead salts of xcex2-diketones such as lead(II) 2,4-pentanedionate. Mixtures of the aforementioned lead compounds may also be employed. The preferred lead compounds are lead(II) oxide, lead(II) aryloxides and lead(II) 2,4-pentanedionate.
Examples of cerium compounds are cerium carboxylates such as cerium(II) acetate, and cerium salts of xcex2-diketones such as cerium(III) 2,4-pentanedionate. Cerium(III) acetylacetonate and mixtures of the aforementioned cerium compounds may also be employed. The preferred cerium compounds are cerium 2,4-pentanedionates.
Examples of titanium compounds are inorganic titanium salts such as titanium(IV) bromide; titanium alkoxides and aryloxides such as titanium(IV) butoxide and titanium(IV) phenoxide; and titanium salts of xcex2-diketones such as titanium(IV) oxide bis(2,4-pentanedionate). Titanium(IV) acetylacetonate and mixtures of the aforementioned titanium compounds may also be employed. The preferred titanium compounds are titanium(IV) alkoxides, aryloxides and 2,4-pentanedionates.
The preferred compounds of other metals are, for the most part, salts of xcex2-diketones and especially 2,4-pentanedionates.
In addition to the aforementioned reactants and catalyst system, it is strongly preferred for a desiccant to be present in the reaction system. The preferred desiccants are non-reactive materials such as molecular sieves, as illustrated by 3-xc3x85ngstrom (hereinafter xe2x80x9c3 xc3x85xe2x80x9d) molecular sieves. They are usually isolated from the other reactants, as by presence in a basket mounted to a stirrer shaft or the like.
Component A is most often present in the amount of about 0.1-10,000 ppm by weight of the appropriate Group VIII metal (usually palladium), based on the total of hydroxyaromatic compound and component C, and component B in the amount of about 1-2,000 mol per mole of the Group VIII metal of component A. Component D, when employed, is generally present in the amount of about 1-200 mole of total metal per equivalent of the Group VIII metal of component A.
The role of component C in the composition and method of the invention is believed to be to increase the degree of dissociation and ionization of the halide anion of component B, perhaps by forming a complex with the cationic portion of said component, although the invention is in no way dependent on this or any other theory of operation. The amount of component C employed will be an amount effective to optimize diaryl carbonate formation, in general by increasing the yield of the desired diaryl carbonate as evidenced, for example, by an increase in xe2x80x9cturnover numberxe2x80x9d; i.e., the number of moles of diary carbonate formed per gram-atom of palladium present. This amount is most often about 1-60% by volume based on the total of hydroxyaromatic compound and component C.
The amount of component C will, however, typically depend to some extent on the complexing ability of the organic compound employed. Crown ethers, for example, have a very high complexing tendency with metal cations. For example, 15-crown-5 complexes efficiently with sodium and 18-crown-6 with potassium. Such compounds may be used in amounts as low as an equimolar amount based on component B. Other compounds useful as component C, such as straight chain polyethers (e.g., diglyme), may be optimally effective at much higher levels, often up to 1-60% by volume based on total polyether and phenol; near the higher end of this range, they can also function as cosolvents. The preferred proportion of any specific material used as component C can be determined by simple experimentation.
The method of the invention is preferably conducted in a reactor in which the hydroxyaromatic compound and catalyst system are charged under pressure of carbon monoxide and oxygen and heated. The reaction pressure is most often within the range of about 1-500 and preferably about 1-150 atm. Gas is usually supplied in proportions of about 1-50 mole percent oxygen with the balance being carbon monoxide, and in any event, outside the explosion range for safety reasons. The gases may be introduced separately or as a mixture. Reaction temperatures in the range of about 60-150xc2x0 C. are typical. In order for the reaction to be as rapid as possible, it is preferred to substantially maintain the total gas pressure and partial pressure of carbon monoxide and oxygen, as described, for example, in U.S. Pat. No. 5,399,734, until conversion of the hydroxyaromatic compound is complete.
The diaryl carbonates produced by the method of the invention may be isolated by conventional techniques. It is often preferred to form and thermally crack an adduct of the diaryl carbonate with the hydroxyaromatic compound, as described in U.S. Pat. Nos. 5,239,106 and 5,312,955.